Titanium or titanium alloy contacting surface

ABSTRACT

The invention essentially relates to the use of a substantially nickel free non-stainless low alloy steel, for manufacturing at least the surface of an item intended to be put into contact with titanium or a titanium alloy, as well as to a method for manufacturing at least the surface of such an article. The invention notably relates the use of this alloy for manufacturing at least the contact surface of a mold for hot forming titanium-based alloys or titanium alloy in the solid-state, or for manufacturing at least the contact surface of tools for densifying and forming bulk titanium alloys or titanium or titanium alloy powders. Items intended to be put in contact with titanium or a titanium alloy may be produced with the invention.

DESCRIPTION

The present invention essentially relates to the use of a substantially nickel-free non-stainless low-alloy steel, for manufacturing at least the surface of an item intended to be put in contact with titanium or titanium alloy in the solid state, as well as to a method for manufacturing at least the surface of such an item with said steel.

STATE OF THE ART

The manufacturing of titanium-based alloy or titanium alloy parts by means of a hot forming mold, or tools, made with different types of steel, is known. Hitherto, at least the portions of the molds or tools in contact with titanium and titanium alloys were made in a selected steel in order to prevent substantial reactions to surface diffusions with titanium and titanium alloys.

Within this framework, the steels used are carbon steels prepared by powder metallurgy (MdP), austenitic stainless steels for recoverable mold components (MdP), maraging steels, and high nickel refractory steels for isothermal, possibly superplastic forming.

However, these steels do not provide sufficient limitation of surface diffusions with titanium and titanium alloys.

Other steel classes are also used for manufacturing molds in contact with titanium alloys. These steels highly alloyed with chromium and molybdenum notably, but nickel-free do not react with titanium. However, these alloys are of the stainless type, this is different from those produced by the present invention.

In both cases, there is diffusion particularly of iron with possible formation of inter-metallic zones, this leads to deteriorations of surface properties. Surface changes to the microstructure are also observed.

To correct this, it is necessary to resort to costly additional operations such as anti-diffusion coating, chemical dissolution of the surface of the parts.

OBJECTS OF THE INVENTION

The main object of the present invention is to solve the new technical problem consisting of selecting for surfaces in contact with titanium and titanium alloys, steel capable of preventing any substantial (measurable) reaction by surface diffusion of a component of this steel, and of iron, in particular, with titanium and titanium alloys.

An object of the invention is still to solve this technical problem according to a simple inexpensive solution, which may be used on an industrial scale for forming titanium or titanium alloy in the solid state.

SUMMARY AND DETAILED DESCRIPTION OF THE INVENTION

For the first time, with the invention, these new aforementioned technical problems may be solved in a simple, inexpensive, reliable and reproducible way, which may be used on an industrial scale.

According to a first aspect, the present invention relates to the use of a substantially nickel-free non-stainless low-alloy steel, for manufacturing at least the surface of an item intended to be put into contact with titanium or a titanium alloy for forming said titanium or titanium alloy in the solid state, for example, either for densifying and/or forming titanium or titanium alloy powders, or for forming bulk titanium or bulk titanium alloy.

According to a first embodiment of the invention, said steel is used for manufacturing at least the contact surface of a mold for hot forming titanium or a titanium alloy in the solid state.

According to a second embodiment of the invention, said steel is used for manufacturing tools for densifying and/or forming titanium or titanium alloy powders.

According to a third embodiment of the invention, said steel is used for manufacturing tools for solid-state forming of bulk titanium or bulk titanium alloy.

Advantageously, forming is not limited to forming “final” parts, but may also comprise the forming of blanks for example, possibly followed by another operation, such as forging, die stamping or rolling.

Advantageously, a low alloy steel is used, comprising less than 10% by weight of alloying elements, in particular, the steel comprises in weight percent:

-   -   nickel: less than or equal to 2.5%, preferably less than or         equal to 0.5%,     -   chromium: between 0.5 and 4%, preferably between 0.8% and 2%,     -   carbon: less than or equal to 1, preferably less than or equal         to 0.4%;     -   balance: iron and unavoidable impurities.

According to an advantageous embodiment, the use is characterized in that the steel comprises from 0.1 to 4% by weight of molybdenum, preferably 0.1 to 2% by weight of molybdenum, still better from 0.15 to 1% by weight of molybdenum.

According to still another advantageous embodiment, the use according to the invention is characterized in that the steel comprises from 0.05 to 4% by weight of vanadium, preferably from 0.05 to 2%, still better from 0.1 to 1%, by weight of vanadium.

According to still another advantageous embodiment of the invention, the use according to any of the preceding claims is characterized in that the steel is 18CD4 grade steel.

According to still another advantageous embodiment of the invention, the use according to any of the preceding claims is characterized in that the steel is 15CDV6 grade steel.

According to a second aspect, the present invention also covers a method for manufacturing at least a surface of an item intended to be put into contact with titanium or a titanium alloy for solid-state forming of said titanium or titanium alloy, for example, either for densifying and/or forming titanium or titanium alloy powders, or for forming bulk titanium or bulk titanium alloy, characterized in that it comprises the manufacturing of said surface with a substantially nickel-free, non-stainless low alloy steel, as defined earlier and in the following description, including examples which are integral parts of the invention, unless stated otherwise. With this, it is possible in particular to prevent any substantial (measurable) reaction by surface diffusion of a component of this steel, and in particular of iron, with titanium and titanium alloys in the solid state. With the steel of the present invention, it is notably possible to prevent sticking and diffusion in titanium or titanium alloys of the constituent elements of the steel of the invention. The surface microstructure is not changed, unlike what is observed for steels of the prior art. Advantageously with the steel of the present invention, it is possible to obtain no measurable contamination of titanium or of the titanium-based alloy in the solid state by elements from the item of the invention.

According to a first embodiment of the invention, the manufacturing method comprises the manufacturing of at least the contact surface of a mold for hot forming titanium and a titanium alloy in the solid state.

According to a second embodiment of the invention, the manufacturing method comprises the manufacturing of tools for densifying and/or forming titanium or titanium alloy powders.

According to a third embodiment of the invention, the manufacturing method comprises the manufacturing of tools for solid-state forming of bulk titanium or bulk titanium alloy.

According to a particular embodiment of these two aspects, the method according to the invention is characterized in that hot forming is performed at a temperature at least equal to 500° C. and less than 1,000° C., preferably less than about 980° C.

According to a particular embodiment, the method is characterized in that molding of TA6V grade titanium parts is performed.

According to still another particular embodiment, the method according to the invention is characterized in that pump impellers in a titanium-based alloy, in particular in a TA6V grade alloy, are manufactured by tools made in said low alloy steel produced by turning or milling.

According to a fourth aspect, the present invention further covers items comprising a contact surface intended to be put in contact with titanium or a titanium alloy for solid-state forming of titanium or a titanium alloy, for example, either for densifying and/or forming titanium or titanium alloy powders, or for the forming of bulk titanium or bulk titanium alloy, characterized in that at least said contact surface of the item is made with a substantially nickel-free non-stainless low alloy steel, notably as defined earlier and in the following description, including examples which are an integral part of the invention, unless stated otherwise, or as obtained by the method as defined earlier and in the following description, including the examples which are an integral part of the invention, unless stated otherwise.

According to a fifth aspect, the present invention further covers tools for densifying and/or forming titanium or titanium alloy powder, characterized in that at least the surface of the tools intended to be put into contact with titanium or the titanium alloy in the solid-state, is made in a non-stainless low alloy steel, as defined earlier and in the following description, including the examples which are an integral part of the invention, unless stated otherwise, or as obtained by the method as defined earlier and in the following description, including the examples which are an integral part of the invention, unless stated otherwise.

According to a sixth aspect, the present invention further covers tools for solid-state forming of bulk titanium or bulk titanium alloy, characterized in that at least the surface of the tools intended to be put into contact with titanium or the titanium alloy in the solid-state, is made in a non-stainless low alloy steel, as defined earlier and in the following description, including the examples which are an integral part of the invention, unless stated otherwise, or as obtained by the method as defined earlier and in the following description, including the examples which are an integral part of the invention, unless stated otherwise.

According to a seventh aspect, the present invention further covers molds for solid-state hot forming of titanium or a titanium alloy, for example either for densifying and/or forming titanium or titanium alloy powders, or for forming bulk titanium or bulk titanium alloy, characterized in that at least the surface of the mold intended to be put into contact with titanium or the titanium alloy in the solid-state, is made in a non-stainless low alloy steel, as defined earlier and in the following description, including the examples which are an integral part of the invention, unless stated otherwise, or as obtained by the method as defined earlier and in the following description, including the examples which are an integral part of the invention, unless stated otherwise.

Advantageously, the invention relates to supporting tools for heat treatment.

According to a particular embodiment, this steel has chromium content between 0.5 and 4% by weight, with nickel content less than 2.5% by weight and carbon content less than 1% by weight.

The steel in the use of the present invention may be implemented by forging, rolling, followed by machining or grinding or a cast steel.

Preferably, this steel is in the annealed condition to improve dimensional reproducibility.

Other objects, characteristics and advantages of the invention will become clearly apparent to one skilled in the art in the light of the description of the use, made with reference to several presently preferred examples of the invention simply given by way of illustration and which would therefore not limit the scope of the invention in any way. In the examples, the percentages are given by weights and the temperatures in degrees Celsius or as the room temperature, the pressure is atmospheric pressure, and the atmosphere is air unless stated otherwise.

EXAMPLES

Examples 1 to 7

Tested Steels of the Invention and Comparative Steels

The following commercially available soft and low alloy steels, of the invention (EX 1, EX 2A and EX 2B) and of the prior art (EX 3A,EX 3B,EX 4 and EX 5), respectively, (in particular grade 15CDV6, is available from THYSSEN FRANCE SA MANUFACTURERS, grade 18CD4 from ASCOMETAL, grade 100HLES from TECPHY FIRMINY) will be submitted to the test of Example 8, and they have the following chemical analyses, in weight percent, reported in Table I, the balance being iron and unavoidable impurities. TABLE 1 Elementary analysis of the steels (wt %) EXAMPLE Grade batch C Mn Si S P Ni Cr Mo V EX 1 = INV I1  18CD4 A4791 0.18 0.764 0.255 0.027 0.012 0.203 1.079 0.23 0.001 EX 2A = INV I2A  15CDV6 78384 0.16 0.97 0.13 0.002 0.007 N.A. 1.41 0.9 0.22 EX 2B = INV I2B  15CDV6  1470 0.16 0.91 0.15 0.001 0.012 N.A. 1.4 0.89 0.25 EX 3A = PRIOR ART A3A XC18 23729 0.2 0.61 0.27 0.028 0.014 0.01 0.03 0.002 N.A. EX 3B = PRIOR ART A3B XC18 69181 0.174 0.69 0.22 0.028 0.012 0.13 0.18 0.03 0.003 EX 4 = PRIOR ART A4 XES 25003 0.12 0.64 0.23 0.03 0.013 N.A. 0.07 N.A. N.A. EX 5 = PRIOR ART A5 100HLES R2417 0.105 0.29 0.29 0.001 0.006 6.8 0.2 0.52 0.085 N.A. = impurities present in very small quantities, for which numerical data are not available.

Example 8 Tests on Specimens from Steels of Examples 1 to 7 Listed in Table I

A Test on Specimens

A.1 Specimens Machined by Milling

Specimens were made from a cylindrical container of about 50 mm in diameter and about 70 mm in height.

The containers were filled with standard TA6V powder and 20*10*10 mm parallelipipedal inserts obtained by milling, made in soft steels of grades XES, XC18 (batch 23729), 18CD4 (batch A4791), 15CDV6 (batch 78384) and 100HLES (batch R2417), were placed inside the powder.

The characteristics of the materials are given for the steel inserts in Table I and for the standard TA6V titanium alloy powder in Table II, respectively.

The containers were densified by hot isostatic compression with a plateau of 4h at 902° C. and 100 MPa. The steel was removed by dissolving it in nitric acid. After densification, the following investigations were carried out:

-   -   on a micrographic section: micrographic examination and         determination of the diffusion profiles at the TA6V-steel         interfaces at the EDAX probe with a scanning microscope.

After dissolution of the steel inserts in nitric acid: determination of the diffusion profile in TA6V,+examination of the geometry of the surface and roughness measurement with the scanning microscope. TABLE II Characteristics of the TA6V powders Elementary analysis (wt %) beta TA6V batch Al C Fe N O2 V H2 transus standard K0122 6.19 0.02 0.17 <0.01 0.016 3.99 0.004 992° C. ELI K0114 6.21 0.008 0.0969 0.0062 0.01447 3.91 0.006 989° C. Production

-   -   spraying with a rotating electrode     -   sieving: 315 μm         Results of the examinations are as follows:         1.XES insert (prior art A4)     -   continuous intermetallic layer of about 5 μm. Diffusion of Fe in         TA6V over about 0.3 mm     -   roughness: Ra=15 μm     -   large indentation of powder grains in the steel     -   microstructure surface alteration         2. XC18 Insert (Prior Art A3A and A3B)     -   continuous intermetallic layer of about 5 μm. Diffusion of Fe in         TA6V over about 0.3 mm.     -   roughness: Ra=9-12 μm     -   significant indentation of powder grains in the steel         3. 18CD4 Insert (Invention I1)     -   absence of any intermetallic layer. No element diffusion from         the steel into the TA6V     -   roughness: Ra=5 μm     -   little indentation of the powder in the steel with diamond tip         formation at the triple points.     -   no microstructure surface alteration         4. 15CDV6 Insert (Invention 12A and 12B)     -   absence of any intermetallic layer. No element diffusion from         the steel into the TA6V     -   roughness: Ra=3-4 μm     -   little indentation of the powder in the steel without any         diamond tip formation at the triple points.     -   no microstructure surface alteration         5. 100HLES Insert (Prior Art A5)     -   irregular intermetallic layer of 0-20 μm depending on the         locations. Diffusion of Fe into TA6V up to 0.3 mm.     -   roughness: Ra=2-3 μm     -   little indentation of the powder in the steel.         A.2 Inserts Produced by Techniques Other than Tool Machining         (Turning or Milling)

The influence of the method for obtaining steel inserts was investigated for the following combinations:

-   -   standard TA6V—18CD4 (batch A4791) machined by water jet cutting     -   standard TA6V—25CD4 molded by lost-wax casting     -   TA6V ELI-15CDV6 (batch 1470) obtained by spark-machining (wire         cutting).

Densification was carried out by so-called Hot Isostatic Compression (HIC), (920° C., 100 MPa, 4h).

The steel-TA6V interfaces were subject to a micrographic examination in order to reveal possible diffusion between both materials. No diffusion was detected for the first two combinations. On the other hand, diffusion in places was revealed on the spark-machined combination TA6V ELI-15CDV6.

B Tests on Parts

B1. ISOPREC® Method:

With the ISOPREC® method, it is possible to obtain shaped parts with good dimensional accuracy for which at least one portion of the surfaces is produced without any machine-finishing operation. The part is produced by densification through HIC (hot isostatic compression) of stainless pre-alloyed powders in a non-stainless steel mold including a cavity, with calculated dimensions so as to obtain the desired final geometry. The mold or the tool is removed by machining and dissolution in nitric acid.

Pump impellers were produced according to the ISOPREC® method, from TA6V and from steel tools produced by turning and milling. The following combinations were produced:

-   -   standard TA6V-XC18 batch 69181     -   standard TA6V 18CD4 batch A4791 (invention I1)     -   TA6V ELI—18CD4 batch A4791 (invention I1)     -   TA6V ELI—15CDV6 batch 78384 (invention 12A)     -   TA6V ELI—15CDV6 batch 1470 (invention 12B)

As a reminder, the characteristics of the materials used are given in Tables I and II.

After densification by HIC (920° C., 100 MPa, 4h) and removal of the steel tool by dissolution in nitric acid, the surfaces were characterized as to their roughness and iron contamination (0.25% detection limit).

The results obtained on specimens were widely confirmed. A slightly larger dispersion of the roughnesses may just be noted because of powder grain size segregation during the filling of the containers.

B2. Manufacturing a Half-Sphere by a Supporting Tool for Heat Treatment:

This method relates to supporting a half-sphere in standard TA6V with a diameter of 550 mm, with a thickness of 5 mm. This half-sphere is brought to about 870° C. in vacuo for one hour, held by a tool with the same recessed geometry, made in 18CD4.

After this treatment, the obtained results show that surface contamination and deformation by iron are absent.

This clearly confirms that this steel may be used for producing a supporting tool for heat treatment, in order to suppress diffusion reactions, in particular by iron.

B3. Method for Manufacturing a Blank by Densification of Powder Followed by Forging:

The manufacturing of a blank by powder densification was achieved followed by forging of this blank.

This blank was manufactured by hot isostatic compression (HIC) densification of a cylindrical container according to FIG. 1, (for example, at 920° C.; 100 MPa, for 4 hours).

The cylindrical container (1) comprises a central core (10), an external container (20), as well as components (40, 41) for sealing the ends.

This cylindrical container (1) has a recess (30) for collecting filling material, such as a powder (50) for example. The thickness of the external container (20) was 3 mm.

By way of illustration, a cylinder with an external diameter of 50 mm, an internal diameter of 30 mm and a length of 200 mm was produced after HIC from this cylindrical container (1).

The central core was in 18CD4 and the external container in XC18.

This container was filled with standard TA6V powder. The obtained cylinder was scalped to a diameter of 45 mm in order to remove the XC18 container and the diffusion zones created upon densification.

The thereby obtained blank was forged at about 900-920° C. by compression of the diameter between plates to a height of 35 mm. The central 18CD4 core was removed by dissolution in nitric acid.

The results showed that the section showed:

-   -   regular deformation of the materials;     -   complete absence of diffusion at the surface of the TA6V         cylinder pre-contacted with the 18CD4 core.

With this, the use of 18CD4 for eliminating diffusion reactions may also be confirmed for manufacturing tools or molds intended to be in contact with TA6V.

The invention comprises all technical means which are technical equivalents of the means described in the specification taken as a whole. In addition, any feature which appears to be novel, with regards to any state of the art, is an integral part of the invention and is claimed as such and in its generality or function. 

1-21. (canceled)
 22. A method for manufacturing at least the surface of an item to be put into contact with titanium or a titanium alloy for solid-state forming of said titanium or titanium alloy, wherein said method comprises the manufacturing of said surface with a substantially nickel-free non-stainless low alloy steel comprising less than 10% by weight of alloy elements.
 23. A Method for manufacturing at least the surface of an item to be put into contact with titanium or a titanium alloy for solid-state forming of said titanium or titanium alloy, wherein said method comprises the manufacturing of said surface with a substantially nickel-free non-stainless low alloy steel comprising less than 10% by weight of alloy elements, said steel comprising in weight percent: nickel: less than or equal to 2.5%, chromium: between 0.5 and 4%, carbon: less than or equal to 1, balance: iron, as well as unavoidable impurities.
 24. The method of claim 23, wherein the steel comprises: nickel: less than or equal to 0.5% chromium: between 0.8% and 2%, carbon: less than or equal to 0.4%, balance: iron, as well as unavoidable impurities.
 25. The method of claim 23, wherein the steel further comprises from 0.1 to 4% by weight of molybdenum.
 26. The method of claim 23, wherein the steel further comprises from 0.15 to 1% by weight of molybdenum.
 27. The method of claim 23, wherein the steel further comprises from 0.05 to 4% by weight of vanadium.
 28. The method of claim 23, wherein the steel further comprises from 0.05 to 2% by weight of vanadium.
 29. The method of claim 23, wherein the steel further comprises from 0.1 to 1% by weight of vanadium.
 30. The method of claim 23, wherein the steel is 18CD4 grade steel.
 31. The method of claim 23, wherein the steel is 15CDV6 grade steel.
 32. The method of claim 23, comprising the manufacturing with said steel of at least the contact surface of a mold for hot forming of titanium or titanium alloy in the solid state.
 33. The method of claim 23, comprising the manufacturing with said steel of tools for densifying or forming titanium or titanium alloy powders.
 34. The method of claim 23, comprising the manufacturing with said steel of tools for solid-state forming of bulk titanium or bulk titanium alloy.
 35. The method of claim 23, wherein hot casting of said titanium or titanium alloy is performed at a temperature equal to 500° C. and less than 1,000° C.
 36. The method of claim 23, comprising molding with said steel of parts in TA6V grade titanium.
 37. The method of claim 23, wherein pump impellers in titanium-based alloy are manufactured by tools having at least their contact surface made with said low alloy steel produced by turning or milling.
 38. The method of claim 23, wherein annealing of the steel is performed in order to improve the dimensional reproducibility.
 39. An item comprising a contact surface to be put into contact with titanium or a titanium alloy for solid-state forming of said titanium or titanium alloy, wherein at least said surface in contact with the item is produced with a substantially nickel-free non-stainless low alloy steel comprising less than 10% by weight of alloy elements.
 40. The item of claim 39, wherein said said steel comprises in weight percent: nickel: less than or equal to 2.5%, chromium: between 0.5 and 4%, carbon: less than or equal to 1, balance: iron, as well as unavoidable impurities.
 41. A tool for densifying and/or forming titanium or titanium alloy powder, wherein at least the surface of the tool to be put into contact with the titanium or titanium alloy in the solid-state, is produced in non-stainless low alloy steel, comprising less than 10% by weight of alloy elements.
 42. The tool of claim 41, wherein said steel comprises in weight percent: nickel: less than or equal to 2.5%, chromium: between 0.5 and 4%, carbon: less than or equal to 1, balance: iron, as well as unavoidable impurities.
 43. The tool of claim 41, wherein the steel further comprises from 0.1 to 4% by weight of molybdenum.
 44. The tool of claim 41, wherein the steel further comprises from 0.05 to 4% by weight of vanadium.
 45. The tool of claim 41, wherein the steel is 18CD4 grade steel.
 46. The tool of claim 41, wherein the steel is 15CDV6 grade steel.
 47. The tool of claim 41, wherein said titanium or titanium alloy is TA6V grade titanium.
 48. A tool for forming bulk titanium or bulk titanium alloy, wherein at least the surface of the tool to be put into contact with titanium or titanium alloy in the solid-state, is produced in non-stainless low alloy steel, comprising less than 10% by weight of alloy elements.
 49. The tool of claim 48, wherein said steel comprises in weight percent: nickel: less than or equal to 2.5%, chromium: between 0.5 and 4%, carbon: less than or equal to 1, balance: iron, as well as unavoidable impurities.
 50. The tool of claim 49, wherein the steel further comprises from 0.1 to 4% by weight of molybdenum.
 51. The tool of claim 50, wherein the steel further comprises from 0.05 to 4% by weight of vanadium.
 52. The tool of claim 48, wherein the steel is 18CD4 grade steel.
 53. The tool of claim 48, wherein the steel is 15CDV6 grade steel.
 54. The tool of claim 48, wherein said titanium or titanium alloy is TA6V grade titanium.
 55. A mold for hot forming of titanium or a titanium alloy in the solid state, wherein at least the surface of the mold intended to be put into contact with the titanium or the titanium alloy in the solid-state is produced in non-stainless low alloy steel, comprising less than 10% by weight of alloy elements.
 56. The mold of claim 55, wherein said steel comprises in weight percent: nickel: less than or equal to 2.5%, chromium: between 0.5 and 4%, carbon: less than or equal to 1, balance: iron, as well as unavoidable impurities.
 57. The mold of claim 56, wherein the steel further comprises from 0.1 to 4% by weight of molybdenum.
 58. The mold of claim 56, wherein the steel further comprises from 0.05 to 4% by weight of vanadium.
 59. The mold of claim 55, wherein the steel is 18CD4 grade steel.
 60. The mold of claim 55, wherein the steel is 15CDV6 grade steel.
 61. The mold of claim 55, wherein said titanium or titanium alloy is TA6V grade titanium. 